1. Field of the Invention
This invention relates generally to a system for optically recording, reproducing, and/or erasing information and, more particularly, to an optical system which is adapted to stabilize feedback control with high accuracy for the positioning means of an information recording, reproducing and/or erasing device.
2. Description of the Prior Art
As is known in the art, there is a conventional information recording and/or reproducing apparatus of the write once type optical disk system wherein semiconductor laser is directly modulated by an information signal and holes are formed in a rotating disk-shaped recording medium, thereby recording the information, and the power of semiconductor laser is reduced to such a level that no hole is formed in the recording medium and is irradiated onto the disk-shaped recording medium, thereby reproducing the information on the basis of the intensity of the light reflected from the recording medium. An example of such read-write type optical systems is shown in Japanese patent application No. 149157/1984 (Japanese patent public disclosure No. 26942/1986) which was filed by the applicant of the present invention. FIG. 1 shows a constitutional diagram of a conventional system for recording and/or reproducing information according to this patent application, in which the portions which are not concerned with the present invention are omitted.
In FIG. 1, reference numeral 1 denotes a light source consisting of, e.g., a semiconductor laser; 2 is a collimator lens for converting the light beams from the light source 1 into the parallel light flux of the beams; 3 a prism for compensating expansion angles of the light beams from the light source 1, which expansion angles vary with the incident angle of the light to the lens 2; 4 an objective lens for condensing the parallel light flux onto a disk 5 on which concentric or spiral guide grooves 6 have been previously formed and which has a recording medium consisting of a reflecting film or the like. The disk 5 is rotated by a disk motor 7. Numeral 8 denotes an objective lens actuator which is constituted by something like a voice coil of a speaker. The actuator 8 moves the objective lens 4 in the radial direction of the disk 5, for the purpose of enabling a light spot focused by the objective lens 4 to be maintained at the center of the guide groove 6. Numeral 9 indicates a quarter-wave plate (a 1/4 plate) for shifting the phase of the light from the light source 1 by 1/4 wavelength and thereby shifting the reflected lights from the disc 5 by the total of 1/2 wavelength for the round trip; 10 denotes a beam splitter for bending the optical path of the reflected phase-shifted light by an angle of about 90.degree. as shown; and 11 a two-split type photodetector consisting of photo sensing devices 11a and 11b for receiving the reflected light and converting it into an electric signal. Further, numeral 12 denotes an arithmetic circuit for calculating the difference between the outputs of the photo sensing devices 11a and 11b of the photodetector 11; 13 is an arithmetic circuit for calculating the sum of those outputs; 14 and 15 amplifiers for amplifying the outputs "a" and "b" of the arithmetic circuits 12 and 13, respectively; and 16 a division circuit for dividing the output "A" of the amplifier 14 by the output "B" of the amplifier 15 and outputting a correction error signal C (=A/B). The components 12 to 16 coact to form a position deviation detecting section 17 for controlling the position of the objective lens 4. Reference numeral 18 denotes a correction circuit for compensating the phase delay or the like of the correction errorsignal "C" ; 19 is a drive circuit for making the actuator 8 operative on the basis of the output form the correction circuit 18; and 20 a drive circuit for modulating the light source 1.
The operation of the conventional system constituted as mentioned above will now be explained with reference to FIG. 1 and an operation explanatory waveform diagram shown in FIG. 2.
The light emitted from the light source 1 is converted into parallel light flux by the collimator lens 2 and is waveform-shaped by the prism 3. Thereafter, the light flux is transmitted through the beam splitter 10 and then the phase of the light is shifted by 1/4 wavelength through the 1/4 wavelength plate 9. Then, the phase shifted light flux is fooused onto the guide groove 6 of the disk 5 by the objective lens 4.
In the case of recording the information onto the disk 5 at this time, the drive circuit 20 directly modulates the light source 1 in accordance with the information signal to be recorded in the groove. Thus, the energy density of the focused light spot is increased to a value above which the density is enough to change the reflection factor of the recording medium in the guide groove 6, or decreased below such density. For example, in the case of recording such an information signal as shown in FIG. 2(A), when the logical level of the signal is "1", the emitting light power from the light source 1 is increased to evaporate the recording medium of the guide groove 6 and the reflection factor in this portion is remarkably reduced and the power is decreased when the logical level is "0" so that the recording medium is not changed, thereby recording the information signal consisting of the signal levels "0" and "1" in the guide groove 6.
On the other hand, in the reproducing mode, the energy density of the focused light spot is maintained below the level at which the recording medium is evaporated. For example, assuming that the information signal as shown in FIG. 2(A) has been recorded in the guide groove 6, the light reflected from this portion is transmitted through the objective lens 4 to the quarter-wave plate 9 where the phase is shifted by 1/4 wavelength. Thereafter, the phase shifted light is bent by an angle of about 90.degree. through the beam splitter 10 and is fed to the photodetector 11. Since the guide groove 6 has a structure which is convex or concave relative to the portion around this groove by an amount of about 1/8 wavelength, the reflected lights are diffracted by the side wall of the guide groove 6, so that the positional deviation between the focused light spot and the guide groove 6 causes an anisotrophy in the reflected lights. Therefore, by deducing the difference between the outputs of the photo sensing devices 11a and 11b of the photodetector 11, the error signal "a" for the tracking control of the objective lens 4 can be obtained. Further, by deducing the sum of the outputs of the photo sensing devices 11a and 11b, an information signal such as that shown in FIG. 2(B) (the output of the arithmetic circuit 13 in the recording mode) can be derived for the information signal shown in FIG. 2(A) in the recording mode, while an information signal such as that shown in FIG. 2(C) (the output of the arithmetic circuit 13 in the reproducing mode) can be obtained in the reproducing mode for the disk 5 on which the information signal as shown in FIG. 2(A) was recorded, respectively.
In addition, the output "A" which is amplified by the amplifier 14 is divided by the output "B" which is similarly amplified by the amplifier 15 in the division circuit 16. The output of the circuit 16 is used as the correction error signal "C" which is transmitted to the correction circuit 18. The corretion circuit 18 compensates, for example, the phase of the error signal, and the corrected signal is delivered to the drive circuit 19 used for the objective lens actuator 8. The objective lens 4 is controlled in a feedback manner so that the light spot focused by the objective lens 4 is located at the center of the guide groove 6. The reason why the output of the division circuit 16 is used as the correction error signal "C" as the input variable for the feedback control of the objective lens 4 is to make it possible to compensate the fluctuation in servo loop gain of the tracking in association with the change of a large amount of light between the recording and reproducing modes, the change in reflection factor of the recording medium in the reproducing mode, and the change in transmittance of the optical system.
The conventional system for recording and/or reproducing information is constituted as described above. Thus, means for recording and/or reproducing an information signal onto/from a recording medium is positioned over the guide groove of the recording medium by the feedback control using a correction error signal "C" derived from an error signal "a".
In general, the guide groove is formed by a discrete apparatus, a so-called master recorder wherein a photo resist on a glass plate whose surface is finished like a mirror is exposed using a light spot whose diameter is smaller than the focused light spot which is used in the signal recording and/or reproducing means for recording and/or reproducing information and then the exposed photo resist is developed.
Therefore, the width of recording pits on the recording medium produced by means for recording and/or reproducing information is larger than that of the guide groove. Thus, there is a problem in that the groove structure is partially destroyed and a positioning error signal for controlling the positioning of the recording and/or reproducing means over the guide groove can not be derived from the portion of the recording pit.
For overcoming this problem, a method whereby a correction error signal which is derived by dividing the error signal by the sum of the outputs of the respective photo sensing devices is utilized (hereinafter, this method is called an auto gain control (AGC) with the meaning that the positioning control loop gain is corrected), but this method is not effective.
This is because when the light spot is located on the recording pit, the sum of the outputs does not become zero due to the reflected lights from the outside portion surrounding the recording pit of the light spot; on the other hand, the error signal becomes zero in principle since the groove structure has been lost.
FIG. 3 shows the results of certain experiments. The width of a guide groove was set at 0.6 .mu.m. A recording pit was formed so as to have a width of about 0.8 .mu.m. Since the pitch of the guide grooves is 1.6 .mu.m, the error signals (in the diagram, since these error signals are used for tracking, they are represented as the amplitude of a tracking sensor) which are obtained when the light spot traverses the guide grooves are a periodic function with a period of 1.6 .mu.m. Therefore, amplitudes of tracking sensors before and after the foregoing AGC were measured, wherein the power of the light at the recording medium is used as a parameter.
It will be found from the results of these experiments that it is hardly possible to expect that the desired effect of the AGC can be achieved.
Since the recording pit length is constant, as the recording frequency rises, the rate at which the guide grooves become lost due to the forming of the recording pits increases and the amplitude of the tracking error signal decreases, namely, the sensitivity deteriorates. The tracking servo positioning control) loop gain decreases and the feedback loop becomes unstable.
This problem becomes a large obstacle in terms of obtaining high-density recording on a recording medium.